Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Include/setobject.h

@@ -1,10 +1,7 @@
-/* Set object interface */
-
 #ifndef Py_SETOBJECT_H
 #define Py_SETOBJECT_H
-#ifdef __cplusplus
-extern "C" {
-#endif
+COSMOPOLITAN_C_START_
+/* clang-format off */
 
 #ifndef Py_LIMITED_API
 
@@ -102,7 +99,5 @@ PyAPI_FUNC(Py_ssize_t) PySet_Size(PyObject *anyset);
     (Py_TYPE(ob) == &PyFrozenSet_Type || \
       PyType_IsSubtype(Py_TYPE(ob), &PyFrozenSet_Type))
 
-#ifdef __cplusplus
-}
-#endif
+COSMOPOLITAN_C_END_
 #endif /* !Py_SETOBJECT_H */